1. Field of the Invention
The present invention relates to a structure of a semiconductor laser. More specifically, the invention relates to a structure of a semiconductor laser capable of low noise operation.
2. Description of the Related Art
Conventionally, a semiconductor laser has been widely employed in information apparatus and information processing apparatus, as a light source of an optical disk unit, and a light source of an optical communication. Particularly, in the recent years, an optical disk medium, such as a DVD (digital versatile disk), a magneto-optic (MO) disk and so forth have been utilized as high density storage devices. In such high density storage devices, the semiconductor laser is used as a light source.
In the semiconductor laser to be employed as an optical pick-up for the device set forth above, a noise should be generated due to reflected optical feed back from the disk surface of the optical disk. Therefore, it is an important task to suppress generation of the noise.
As one method for reducing generation of the noise is to drive the semiconductor laser at high frequency. This utilizes the fact that pulsation spectrum becomes multi-mode by high frequency driving to reduce influence of the optical feed back. However, in this case, a high frequency superimposing module becomes necessary increasing the cost. Also, a problem (problem of EMC) is encountered due to generation of electromagnetic noise.
A self-pulsation laser exhibits low noise characteristics similar to the high frequency driving. Also, this type of laser is superior for low cost and no possibility of generation of the electromagnetic noise. Therefore, there is a strong demand for development of practically useful self-pulsation laser of low threshold current and low drive current, and can perform self-pulsation with reliability for long period.
The operation of self-pulsation can be obtained by introducing a saturable light absorbing body within a laser resonator and controlling a saturable light absorbing amount. Such self-pulsation and the laser structure therefor has been reported in Extended Abstract of 18th Conference on Solid State Devices and Material (1986), pp 153, No. D-1-2 and Preliminary Reports of 11th Semiconductor Laser Symposium (1994) pp. 21. These semiconductor lasers take active region on the side of mesa-stripe as the saturable light absorbing layer. This semiconductor laser has large light absorption on the side of the laser light emitting portion to results in large astigmatism of 10 .mu.m to 50 .mu.m.
On the other hand, there has been proposed a semiconductor laser element, in which the saturable light absorbing layer is introduced in parallel to the active layer, namely in a part of a clad layer. As this type of semiconductor laser element, AlGaAs self-pulsation laser has been proposed (Japanese Unexamined Patent Publication (Kokai) No. Heisei 6-196810).
Hereinafter, discussion will be given for the conventional self-pulsation semiconductor laser as disclosed in Japanese Unexamined Patent Publication No. Heisei 6-196810.
FIG. 1 is a section showing this conventional self-pulsation semiconductor laser element. The shown semiconductor laser is constructed with an n-type GaAs substrate 102. An n-type first Al.sub.x Ga.sub.1-x As clad layer 103 is formed on the GaAs substrate 102 and has a composition ratio x of 0.51. An n-type first saturable light absorbing layer 104 is formed on the Al.sub.x Ga.sub.1-x As clad layer 103 in a thickness of 0.01 .mu.m to 0.04 .mu.m, which has a carrier concentration of 2.times.10.sup.17 cm.sup.-3 to 5.times.10.sup.17 cm.sup.-3, and composition ratio x of 0.12 to 0.14. An n-type second Al.sub.x Ga.sub.1-x As clad layer 105 is formed on the n-type first saturable light absorbing layer 104 in a thickness of 0.27 to 0.33 .mu.m and has a composition ratio x of 0.51. An Al.sub.x Ga.sub.1-x As active layer 106 is formed on the n-type second Al.sub.x Ga.sub.1-x As clad layer 105 in a thickness of 0.08 .mu.m and has a composition ratio x of 0.13. A third p-type Al.sub.x Ga.sub.1-x As clad layer 107 is formed on the active layer 106 in a thickness of 0.27 .mu.m to 0.33 .mu.m and has the composition ratio x of 0.51. A second p-type saturable light absorbing layer 108 consisting of p-type Al.sub.x Ga.sub.1-x As is formed on the third Al.sub.x Ga.sub.1-x As clad layer 107 in a thickness of 0.01 .mu.m to 0.04 .mu.m and has a carrier concentration of 4.times.10.sup.17 cm.sup.-3 to 2.times.10.sup.18 cm.sup.-3 and the composition ratio x of 0.12 to 0.14. A p-type fourth Al.sub.x Ga.sub.1-x As clad layer 109 is formed on the second saturable light absorbing layer 108 and has the composition ratio x of 0.51. A p-type GaAs cap layer 110 is formed on the upper surface of a ridge portion of the p-type fourth Al.sub.x Ga.sub.1-x As clad layer 109. An n-type GaAs current block layer 112 is formed in a portion where a part of the p-type fourth Al.sub.x Ga.sub.1-x As clad layer 109 and the p-type GaAs cap layer 110 is removed. A p-type GaAs contact layer 111 is formed over the p-type GaAs cap layer 110 and the n-type GaAs current blocking layer 112. A p-type electrode 113 is formed on the p-type GaAs contact layer 111, and an n-type electrode 101 is formed on the lower surface of the n-type GaAs substrate 102.
Here, by appropriately selecting the composition ratio x and layer thickness of the n-type first saturable light absorbing layer 104 and composition ratio x and the layer thickness of the p-type second saturable light absorbing layer 108, low optical feed back noise characteristics by the self-pulsation can be obtained. This laser has a superior feature of low astigmatism, and suitable for mounting on the optical disk unit.
However, in the conventional semiconductor laser set forth above, in the saturable light absorbing layer for generating the self-pulsation, large light absorbing amount is necessary to increase threshold gain for laser pulsation, which results in rising of threshold current and drive current.
Particularly, in AlGaInP red laser, as a result of high threshold current, difficulty in obtaining high reliability can be encountered.